Field of the Invention
The invention relates to a process for operating a high temperature fuel cell installation and to a high temperature fuel cell installation.
It is known, that during the electrolysis of water, water molecules are decomposed by electric current into hydrogen (H.sub.2) and oxygen (O.sub.2). In a fuel cell, that process takes place in reverse. Electric current is produced with high efficiency by the electrochemical combination of hydrogen (H.sub.2) and oxygen (O.sub.2) to form water. If pure hydrogen (H.sub.2) is used as combustion gas, the process takes place without the emission of pollutants and carbon dioxide (CO.sub.2). Even with a technical combustion gas, for example natural gas or coal gas, and with air (which may additionally be enriched with oxygen (O.sub.2)) instead of pure oxygen (O.sub.2), a fuel cell produces considerably less pollutants and less carbon dioxide (CO.sub.2) than other forms of energy production which operate by using fossil energy sources.
The technical implementation of the fuel cell principle has given rise to a variety of solutions, and more precisely with different electrolytes and with operating temperatures of between 80.degree. C. and 1000.degree. C.
Depending on their operating temperature, fuel cells are divided into low, medium and high temperature fuel cells, which in turn differ through various technical embodiments.
A high temperature fuel cell block (a fuel cell block is also referred to a "stack" in the specialist literature) is generally composed of a multitude of high temperature fuel cells which are of planar construction and are stacked on one another. Fuel cell installations including at least one high temperature fuel cell block are, for example, known from German Patent DE 195 23 973 C1, corresponding to U.S. application Ser. No. 09/001,043, filed Dec. 30, 1997; German Patent DE 195 23 972 C1; and German Published, Non-Prosecuted Patent Application DE 195 14 469 A1.
A high temperature fuel cell installation is operated with a high constant operating temperature of, for example, in excess of 900.degree. C. For that purpose, the installation must be supplied with additional heat, in order to achieve the operating temperature before operation or to hold the required operating temperature during brief interruptions in operation.
A further problem is the need to use the working medium efficiently during operation of the high temperature fuel cell block. In order to make it possible to operate the high temperature fuel cell block with high efficiency, the working medium needs to be supplied in excess. It is only through an excess of working medium that it is possible to guarantee that the active surfaces of the high temperature fuel cells are provided with enough working medium. An unavoidable consequence of operating the high temperature fuel cell block with excess working medium is that, after the electrochemical reaction has taken place, there is still some working medium present in the waste gas or off-gas at the outlet of the high temperature fuel cell block. Put another way, the working medium is not fully consumed in the high temperature fuel cell block. Some of it emerges unused, and that impairs efficiency.
German Published, Non-Prosecuted Patent Application DE 41 37 968 A1 discloses a heat exchange device in which the waste gas from a high temperature fuel cell block is fed to an expansion turbine. In that case, the waste gas from the high temperature fuel cell block contains unconsumed air from the cathode waste gas and unconsumed hydrogen from the anode waste gas. In order to heat the waste gas to the input temperature of the expansion turbine, the waste gas is guided before feeding through a heat exchanger integrated in the high temperature fuel cell block. After having been fed into the turbine, the waste gas is expanded therein. That means that only the heat content of the waste gas is used for the production of energy in the turbine, but not actually the components of the waste gas, for example hydrogen.